1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to a surface mode LCD such as a pi-cell device or a splay-bend device (SBD).
2. Description of the Related Art
The term xe2x80x9csurface mode LCDxe2x80x9d as used herein means an LCD in which the optical change caused by varying the electric field across the liquid crystal layer occurs primarily in layers in the vicinity of a substrate of the liquid crystal. Examples of surface mode LCDs are the pi-cell and the splay-bend device, although other types of surface mode LCDs are known. Surface mode LCDs are disclosed in xe2x80x9cSov. J. Quantum Electronicsxe2x80x9d, 1973, Vol 3, p78-79.
The pi-cell (otherwise known as an xe2x80x9coptically compensated birefringent devicexe2x80x9d or OCB) is described in xe2x80x9cMol. Cryst. Liq. Cryst.xe2x80x9d, 1984, Vol 113, p329-339, and in U.S. Pat. No. 4,635,051. The structure of a pi-cell is schematically illustrated in FIG. 1. The device comprises transparent substrates 1, 1xe2x80x2 on which are disposed alignment layers 2, 2xe2x80x2. A layer of nematic liquid crystal 3 is disposed between the substrates 1, 1xe2x80x2.
The alignment layers 2, 2xe2x80x2 create parallel alignment of the liquid crystal molecules in the liquid crystal layer 3 at its boundaries with the alignment layers 2, 2xe2x80x2. This can be achieved by using parallel-rubbed polyamide alignment layers.
Addressing electrodes (not shown) are provided on the substrates 1, 1xe2x80x2, so that an electric field can be applied to selected areas of the liquid crystal layer. The liquid crystal layer 3 is placed between linear polarizers 4, 4xe2x80x2, whose transmission axes are crossed with one another and are at 45xc2x0 to the optic axis of the liquid crystal layer.
A retarder 5 with its optic axis perpendicular to the optic axis of the liquid crystal layer, may optionally be provided to compensate for the retardation of the liquid crystal layer. The retarder lowers the required range for the operating voltage by allowing zero retardation of the LCD to be achieved at a finite voltage across the liquid crystal layer.
FIG. 1 shows a transmissive LCD. A pi-call can also be embodied as a reflective device by providing a reflector below the liquid crystal layer, possibly by making the addressing electrode on the lower substrate a reflective electrode. The lower polarizer 4xe2x80x2 is not required with a reflective pi-cell.
The principle of operation of the pi-cell device is illustrated in FIGS. 2A to 2D.
When no electric field is applied across the liquid crystal layer, the liquid crystal is in an H-state (homogenous state or splay state), in which the liquid crystal molecules in the center of the liquid crystal layer are substantially parallel to the substrates. This is shown in FIG. 2A. The short lines in the figures represent the director of the liquid crystal molecules.
When an electric field greater than a threshold value is applied across the liquid crystal layer, the liquid crystal molecules adopt a V-state (or a band state). In this state, the liquid crystal molecules in the center of the liquid crystal layer are substantially perpendicular to the substrates. FIG. 2C shows a first V-state which occurs at a low applied voltage across the liquid crystal layer, and FIG. 2D shows a second V-state which occurs when a higher voltage is applied across the liquid crystal layer. The pi-cell is operated by switching the liquid crystal layer between the first, low voltage V-state and the second, higher voltage V-state.
If the electric field across the liquid crystal layer should be reduced below the threshold value, the liquid crystal layer will relax to the H-state of FIG. 2A; in order to re-commence operation of the device, it is necessary to put the liquid crystal layer back into the V-state. This generally requires a large applied voltage, owing to the low pre-tilt of the liquid crystal molecules. The pre-tilt is usually below 45xc2x0 and typically between 2 and 10xc2x0 so as to provide sufficient optical modulation and fast switching between the two V-states (for instance of the order of a millisecond or less).
One problem with known OCB devices is the difficulty of nucleating and stabilising the V-state, which is topologically distinct from the H-state. One prior art technique is described in UK Patent Application 9521043.1/2 306 228. In this prior art technique, the V-state is nucleated under the application of a high voltage, and is stabilized by the polymerization of a network whilst a high voltage is applied. This prior art technique is, however, unsuitable for use in active matrix devices, since it is difficult to apply voltages having the required magnitude in a TFT panel. A further disadvantage is that the in-situ polymerization can lead to ionic contamination of the liquid crystal layer, and result in image sticking.
The SBD device, which is also a surface mode device, is described in UK Patent Application No. 9712378.0/2 326 245. The structure of an SBD device is generally similar to that of a pi-cell, except that the alignment layers in an SBD device have a high pre-tilt whereas the alignment layers in a pi cell have a low pre-tilt. An SBD device uses a liquid crystal material with a negative di-electric anisotropy, whereas a pi-cell uses a liquid crystal material having a positive di-electric anisotropy.
The principle of operation of an SBD is analogous to that of a pi-cell. When no voltage is applied across the liquid crystal layer of an SBD, the stable liquid crystal state is a V-state. When an electric field greater than a threshold value is applied across the liquid crystal layer, an H-state becomes stable. The SBD is operated by switching the liquid crystal between a first H-state which occurs at a low applied voltage across the liquid crystal layer and a second H-state which occurs when a higher voltage is applied across the liquid crystal layer. If the electric field across the liquid crystal layer is reduced below the threshold value, the liquid crystal Will relax into the V-state and it will be necessary to put the liquid crystal back into the H-state before operation can be re-commenced.
The high pre-tilt alignment layers required for an SBD can be produced, for example, by the photo-polymerization of a mixture of reactive mesogens.
SID 97 Digest, page 739, discloses a method of promoting nucleation of the V-state in a pi-cell. Voltages of the order of 20 V are applied across the liquid crystal layer to switch the liquid crystal from the H-state to the V-state. However, it is difficult to provide voltages of this magnitude in a TFT (thin film transistor) substrate.
Japanese published Patent Application JP-A-9 90432 (Toshiba) discloses the provision of nucleation sites within a pi-cell panel. The nucleation sites are provided by including spacer balls or pillars within the pi-cell panel, and cooling the liquid crystal material from an isotropic phase to a nematic phase while an electric field is applied across the panel. This results in some of the spacer balls/pillars acting as nucleation sites for growth of the V-state into the existing H-state. This prior art has a number of disadvantages. Firstly, it requires additional process steps during fabrication of the panel, since it is necessary to align the liquid crystal molecules under the influence of an applied electric field. These additional process steps complicate the fabrication of the panel. Secondly, some spacer balls/pillars can cause the H-state to form in the desired V-state, thus destabilising the operating state of the panel.
Miwa et al disclose, in IDW 97-Digest page 85, a method of maintaining the stability of a V-state in a pi-cell. A resetting period is provided within each frame, and the high voltage V-state is addressed in this period. This prevents the liquid crystal layer relaxing to the H-state when low driving voltages are applied. This does not, however, address the initial nucleation of the V-state from the H-state.
U.S. Pat. No. 4,566,758 discloses a surface mode nematic liquid crystal display device in which the liquid crystal layer contains a chiral dopant. When no voltage is applied across the liquid crystal layer, the liquid crystal relaxes to a twist state rather than to an H-state. The problems associated with nucleating the V-state are reduced or even eliminated, since the twist state is topologically equivalent to the V-state. However, this approach requires that the ratio of the thickness d of the liquid crystal layer to the pitch p of the twist of the liquid crystal molecules must be d/p greater than 0.25, and such a high d/p value is known to reduce the brightness of the display. It is not possible to reduce d/p below 0.25 since, in order to obtain a 180xc2x0 twist, it is necessary to bias the pitch of the liquid crystal molecules enough to favor the twist angle of 180xc2x0 over a twist angle of 0xc2x0, and this requires the value of d/p to be greater than 0.25. (It is necessary for the liquid crystal to conform to the rubbing directions of the alignment films on the two substrates, so that the liquid crystal layer in a pi-cell geometry is constrained to have a twist of 0xc2x0, 180xc2x0, 360xc2x0, etc.)
Co-pending UK Patent Application No. 9822762.2 (GB 2 343 011) discloses a surface mode liquid crystal display device in which the liquid crystal layer contains nucleation regions in which the desired operating state is stabilized. These nucleation regions are obtained by providing regions of different pre-tilt angle on at least one of the substrates Such a device has the disadvantage that additional processing steps are required in order to provide the regions of different pre-tilt angle that produce the nucleation regions.
As is well known in the field of liquid crystals, the H- and V-states of a pi-cell have the property that they are topologically in-equivalent i.e. application of voltage to a pi-cell in the H-state cannot produce a smooth and continuous transformation of the pi-cell into a V-state (and vice versa). Instead the transformation requires that alignment defects (alternatively known as xe2x80x98discontinuitiesxe2x80x99, xe2x80x98singularitiesxe2x80x99 or xe2x80x98disclinationsxe2x80x99) must form (or xe2x80x98nucleatexe2x80x99) in the liquid crystal before the transformation can occur. Observation of a pi-cell (using for example, a polarizing optical microscope) undergoing a transformation from an H-state to a V-state allows the nucleation of such defects to be clearly observed, and their movement through the liquid crystal to be observed as the V-state grows to replace the H-state.
The H- and T-states (twisted state) are also topologically in-equivalent, that is, a transformation from the T- to H-state requires the nucleation and movement of defects within the liquid crystal.
The nucleation of the defects in the liquid crystal alignment that are required to produce transitions between topologically in-equivalent states can be difficult to reliably achieve and control. It is an object of the current patent to reliably and controllably allow the transformation of a pi-cell from one state (e.g., an H-state) into another state (i.e., the V- or T-state).
The V-state and the T-state of a pi-cell are topologically equivalent. So for example, if the voltage on a uniform pi-cell initially in the T-state is smoothly increased, the pi-cell will undergo a uniform and continuous transformation into the V-state, without any requirement for the nucleation and movement of defects. The configurations of two V-states at different voltages are also topologically equivalent, that is a V-state configuration at either voltage can be continuously transformed into the V-state configuration at the second voltage, by the act of varying applied voltage.
The present invention provides a liquid crystal display device comprising: a chiral liquid crystal layer disposed between first and second substrates, and means for applying a voltage across the liquid crystal layer; wherein a first region of the liquid crystal layer is an active region for display and a second region of the liquid crystal layer is a nucleation region for generating a desired liquid crystal state in the first region when a voltage is applied across the liquid crystal layer, and wherein the ratio of the thickness d of the liquid crystal layer to the pitch p of the liquid crystal material has a first value (d/p)A in the first region of the liquid crystal layer and has a second value (d/p)N different from the first value in the second region of the liquid crystal layer.
The second region acts as a nucleation region. The stable state in the second region when no voltage is applied across the liquid crystal layer is chosen such that, when a voltage is applied across the liquid crystal layer, the desired stable state is generated in the active region easily and repeatably.
The value (d/p)N of the ratio of the thickness d of the liquid crystal layer to the pitch p of the liquid crystal material in the second region of the liquid crystal layer may be selected such that, when no voltage is applied across the liquid crystal layer, the liquid crystal state stable in the second region of the liquid crystal layer is topologically equivalent to the desired liquid crystal state. This facilitates the process of nucleating the desired stable state in the active region.
The first twist angle may be different by 180xc2x0 from the second twist angle.
The value of the ratio of the thickness d of the liquid crystal layer to the pitch p of the liquid crystal material in the first and second regions of the liquid crystal layer may be selected such that, when no voltage is applied across the liquid crystal layer, a first liquid crystal state having a first twist angle is stable in the first region of the liquid crystal layer, and a second liquid crystal state having a second twist angle different from the first twist angle is stable in the second region of the liquid crystal layer.
The value (d/p)N of the ratio of the thickness d of the liquid crystal layer to the pitch p of the liquid crystal material in the second region of the liquid crystal layer may be greater than the value (d/p)A of the ratio of the thickness d of the liquid crystal layer to the pitch p of the liquid crystal molecules in the first region of the liquid crystal layer. The first liquid crystal state may be a 0xc2x0 twist state and the second liquid crystal state may be a 180xc2x0 twist state. The 0xc2x0 twist state may be an H-state.
As noted above, the 180xc2x0 twist state is topologically equivalent to the desired operating states of a pi-cell. When the invention is applied to a pi-cell, when a voltage is applied across the liquid crystal layer, the desired operating state will grow from the 180xc2x0 twist state contained in the nucleation region.
An alignment direction on the first substrate may be parallel to an alignment direction on the second substrate, and the values of the ratio of the thickness d of the liquid crystal layer to the pitch p of the liquid crystal molecules in the first and second regions of the liquid crystal layer may be selected such that (d/p)A less than 0.25 and 0.25xe2x89xa6(d/p)Nxe2x89xa60.75. These alignment directions will stabilize the 0xc2x0 twist state in the first region and the 180xc2x0 twist state in the nucleation region.
The ratio (d/p)A of the thickness d of the liquid crystal layer to the pitch p of the liquid crystal material in the first region of the liquid crystal layer may be less than 0.125, and it may be less than 0.1. Use of a low d/p ratio means that the brightness of the active region will not be significantly reduced.
The device may be a surface mode liquid crystal display device. The device may be a pi-cell. The desired liquid crystal state in the active region may be a V-state.
The device may be a reflective liquid crystal display device. The second liquid crystal layer region may be disposed at a via-hole. It is normal for a reflective liquid crystal display device to incorporate via holes, for example to enable electrical connection to a reflective electrode. The thickness of the liquid crystal layer in a via-hole region will be greater than the thickness of the liquid crystal layer elsewhere, and so the thickness-to-pitch ratio will be greater in the via hole region than elsewhere. Thus a via-hole region can conveniently be used as a nucleation region in a liquid crystal display device according to the present invention. In this embodiment, no additional processing steps are required to obtain a region of increased thickness-to-pitch ratio of the liquid crystal layer to act as the nucleation region.
The device may be a transmissive and reflective liquid crystal display device. The second region may be provided in a transmissive region in the transmissive and reflective liquid crystal display device. The nucleation region and the active region may be provided in a display region in the transmissive and reflective liquid crystal display device.
The twist of the liquid crystal layer in the second region of the liquid crystal layer may be non-zero and may be opposite to the natural twist of the liquid crystal molecules, and the value (d/p)N of the ratio of the thickness d of the liquid crystal layer to the pitch p of the liquid crystal molecules in the second liquid crystal region may be smaller than the value (d/p)A of the ratio of the thickness d of the liquid crystal layer to the pitch p of the liquid crystal molecules in the first region of the liquid crystal layer.
An alignment direction on the first substrate may be at an angle xcfx86 to an alignment direction on the second substrate, and the values of the ratio of the thickness d of the liquid crystal layer to the pitch p of the liquid crystal molecules in the first and second regions of the liquid crystal layer may be selected such that:                     (                  d          p                )            a        ≤                  φ                  2          ⁢          π                    -              1        4              ;            and      ⁢              xe2x80x83            ⁢                        (                      d                          p              ⁢                              xe2x80x83                                              )                N               greater than                   φ                  2          ⁢          π                    -              1        4            
The pitch pA of the liquid crystal molecules in the first region of the liquid crystal layer may not be equal to the pitch pN of the liquid crystal molecules in the second region of the liquid crystal layer.
The thickness dA of the liquid crystal layer in the first region of the liquid crystal layer may not be equal to the thickness dN of the liquid crystal layer in the second region of the liquid crystal layer.
The voltage applying means may be arranged to apply the voltage across the first and second regions. For example, the first region may surround the second region. Alternatively, the voltage applying means may be arranged not to apply the voltage across the second region. For example, the second region may be disposed in an inter-pixel gap.
A voltage may be applied to the nucleation region for substantially the same amount of time as an amount of time during which a voltage is applied to a display region of the liquid crystal display device.
The liquid crystal display device may further comprise a storage capacitor. The nucleation region may be provided in the vicinity of the storage capacitor.
The present invention also provides a liquid crystal display device comprising: a liquid crystal layer disposed between first and second substrates, and means for applying a voltage across the liquid crystal layer. The liquid crystal layer includes a first region having a first thickness, and a second region adjacent to the first region having a second thickness. A first state of the liquid crystal layer in the first region and a second state of the liquid crystal layer in the second region coexist under a predetermined condition. The first state is topologically different from the second state.
The predetermined condition may be the absence of an applied voltage across the liquid crystal layer.
The first state may be the H-state and the second state may be a T-state.
A tilt angle of the liquid crystal layer may be defined by the liquid crystal layer and the first substrate. A tilt angle of the liquid crystal layer in the first region may be substantially equal to a tilt angle of the liquid crystal layer in the second region.
The first state and the second state may be the V-states in the presence of an applied voltage across the liquid crystal layer.